1. Field of the Invention
The present invention relates to a cathode ray tube, and more particularly to a cathode ray tube characterized by plate-like grid electrodes constituting an electron gun thereof and a method for manufacturing the same.
2. Description of the Related Art
A color cathode ray tube used as a video tube or a monitor tube of an information terminal includes an electron gun irradiating a plurality (generally three) of electron beams at one end of an evacuated envelope. A phosphor screen coated with a phosphor layer made of a plurality of colors (generally three colors) is formed on an inner surface of the other end of the color cathode ray tube. A shadow mask which constitutes a color selection electrode is disposed close to the phosphor screen. By scanning a plurality of electron beams irradiated from the electron gun two-dimensionally by a magnetic field generated by a deflection yoke mounted on an outer portion of the evacuated envelope, given images can be displayed.
The above-mentioned electron gun further includes electron beam generating means which irradiates three electron beams (a so-called triode portion), focusing and accelerating means for focusing and accelerating these electron beams toward the phosphor screen, and a shield cup for shielding a leakage magnetic field from the deflection yoke at a final electrode side.
This type of electron gun is generally constituted by arranging a plurality of cylindrical grid electrodes and a plurality of plate-like grid electrodes in a tube axis direction. These plate-like grid electrodes are formed by punching a plate material having a thickness greater than a thickness of a material for forming the cylindrical grid electrodes using a press machine.
This type of punching of the plate-like grid electrode is comprised of the first step which sequentially punches out a large number of same kinds of electrodes which are connected as a group from a strip-like metal plate and the subsequent step which cuts and separates these electrodes as individual plate-like grid electrodes.
FIG. 10 is an explanatory view showing a state in which same kinds of plate-like grid electrodes are formed as a group by the first punching. Numeral 31 indicates individual plate-like grid electrodes. Each plate-like grid electrode 31 has an approximately rectangular planar shape with a curvature at corners thereof. The plate-like grid electrode 31 is provided with tabs 36 formed on a pair of one parallel sides of the rectangular shape. The tabs 36 are embedded into beading glass for fixing the plate-like grid electrode 31. Further, the plate-like grid electrode 31 is provided with three electron beam apertures 30a, 30b, 30c which are arranged in an in-line array along the longitudinal center line of the rectangular shape.
In the state that a first punching step is completed, connecting portions 33 are formed in a region adjacent to a pair of the other sides of the plate-like grid electrode 31. In a subsequent punching step, these connecting portions 33 are removed along a punching line 35 so as to cut and separate the electrodes formed as a group into individual unit electrodes. When the unit electrodes are cut and separated in the above-mentioned subsequent punching step, protruding portions 38 are formed on end portions of respective electrodes.
In the above-mentioned conventional punching, when the connecting portions 33 are punched out along the punching line 35, burrs are formed on end portions of the protruding portions 38.
FIG. 11 is an explanatory view showing a profile-line crossing angle made by a profile line formed by the conventional first punching step and a profile line formed by the conventional subsequent punching step. In the drawing, numeral 34 indicates the profile line punched by the first punching step and numeral 35 indicates a separation line punched by the subsequent punching step.
The connecting portions 33 which remain after the first punching step are separated along the separation lines 35 in the subsequent punching step. The separation lines 35 are indicated as curves bulged toward the connecting portion 33 side to be removed. On the other hand, the profile lines 34 are formed in the region of the connecting portion 33 such that they approach in the direction of an imaginary center line 301 of electron beam apertures having an in-line array and formed on the plate-like grid electrode. As a result, the crossing angle a made by the profile line 34 formed by the first punching step and the punching line, that is, the separation line 35 formed by the subsequent punching has becomes not less than 135 degrees as shown in the drawing.
Further, it has been thought that by making the crossing angle a as large as possible, the electric discharge in the inside of the color cathode ray tube can be suppressed. However, when the crossing angle a is increased, the burrs at the crossing portion become large and the electric discharge derived from the burrs is increased. Accordingly, a lot of time has been required for performing the barrel polishing.
FIG. 12A to FIG. 12C are partial views for explaining the states of the protruding portions of the plate-like grid electrode after removing the connecting portions by the above-mentioned subsequent punching. FIG. 12A is a plan view, FIG. 12B is a cross-sectional view taken along a line Axe2x80x94A of FIG. 12A and FIG. 12C is a side view of the electrode 31.
In this kind of punching, as shown in FIG. 11, when the punching is performed with the crossing angle a between the profile line 34 and the separation line 35 set to not less than 135 degrees, due to the frictional force generated between dies of the press working machine (a punch and a die), the material slips away toward the outside at the crossing portion of both lines. Due to such an action, the burrs 37 which extend in the planar direction of the electrode as shown in FIG. 12A are formed. Further, burrs 38 having an angle against the surface of the electrode as shown in FIG. 12B are formed in the punching cross-sectional direction. The burrs 38 are particularly large or outstanding in the vicinity of the crossing points as shown in FIG. 12C. Further, these burrs are liable to be formed at the time of forming electrodes having a thickness of not less than 0.5 mm.
When these burrs are present on the electrodess, in assembling the electrodes into an electron gun, the burrs become equal to or exceed the tolerance so that the assembling becomes impossible or it gives rise to the lowering of the assembling accuracy. Further, this brings about problems in terms of enhancing the quality of the color cathode ray tube including a following problem. That is, during the operation of the electron gun, the electric field is converged on these burrs so that a spark is generated resulting in the rupture of the electron gun or the rupture of the cathode ray tube.
Conventionally, these burrs are removed by a suitable polishing method such as the barrel polishing. However, on all of the products (plate-like grid electrodes) which are subjected to such a polishing, an inspection must be performed whether the burrs are sufficiently removed at a level necessary for allowing the assembling of these electrodes into the electron gun. This forms a bottleneck in the reduction of man-hours and has been one of the tasks to be solved for reducing the overall manufacturing cost of color cathode ray tubes. Further, the barrel polishing step requires a lot of time for removing burrs.
The present invention include an angle at a crossing portion which is made by a profile line formed by the first punching and a separation line formed by the subsequent punching to not less than 70 degrees and not more than 110 degrees.
The present invention is further constituted by a method of manufacturing color cathode ray tubes which includes a first punching step for forming profile lines in a state that a plurality of plate-like grid electrodes are connected by a connecting portion and a subsequent punching step for forming a cutting line which crosses the profile line at an angle of not less than 70 degrees and not more than 110 degrees.
By performing such a method, the slipping away of the material at the time of punching for removing connecting portions can be minimized and the generation of burrs can be reduced.